Abstract
BackgroundErwinia amylovora is generally considered to be a homogeneous species in terms of phenotypic and genetic features. However, strains show variation in their virulence, particularly on hosts with different susceptibility to fire blight. We applied the RNA-seq technique to elucidate transcriptome-level changes of the lowly virulent E. amylovora 650 strain during infection of shoots of susceptible (Idared) and resistant (Free Redstar) apple cultivars.ResultsThe highest number of differentially expressed E. amylovora genes between the two apple genotypes was observed at 24 h after inoculation. Six days after inoculation, only a few bacterial genes were differentially expressed in the susceptible and resistant apple cultivars. The analysis of differentially expressed gene functions showed that generally, higher expression of genes related to stress response and defence against toxic compounds was observed in Free Redstar. Also in this cultivar, higher expression of flagellar genes (FlaI), which are recognized as PAMP (pathogen-associated molecular pattern) by the innate immune systems of plants, was noted. Additionally, several genes that have not yet been proven to play a role in the pathogenic abilities of E. amylovora were found to be differentially expressed in the two apple cultivars.ConclusionsThis RNA-seq analysis generated a novel dataset describing the transcriptional response of the lowly virulent strain of E. amylovora in susceptible and resistant apple cultivar. Most genes were regulated in the same way in both apple cultivars, but there were also some cultivar-specific responses suggesting that the environment in Free Redstar is more stressful for bacteria what can be the reason of their inability to infect of this cultivar. Among genes with the highest fold change in expression between experimental combinations or with the highest transcript abundance, there are many genes without ascribed functions, which have never been tested for their role in pathogenicity. Overall, this study provides the first transcriptional profile by RNA-seq of E. amylovora during infection of a host plant and insights into the transcriptional response of this pathogen in the environments of susceptible and resistant apple plants.
Highlights
Introduction ofE. amylovora cells to apple tree tissue influenced the metabolic pathways of the bacteria
We found other genes known to play or possibly play a role in pathogenicity that were more highly expressed in Idared, including hrpA1, belonging to Type 3 secretion system (T3SS), galF, a precursor of amylovoran formation, srlA, sorbitol permease, three (EAMY_1021, EAMY_1023, EAMY_1024) out of five genes involved in biosynthesis of 6-thioguanine and argD, a gene coding for the N-acetylornithine aminotransferase enzyme, which is involved in the production of the amino acid arginine, and mutation in this gene causes arginine auxotrophy, nonpathogenicity in apples, and reduced virulence in pears [47]
The only significant differences in expression of previously recognized genes crucial for pathogenesis were observed for flagellar genes (FlaI), which had higher expression in Free Redstar, and hrpA, three out of five genes involved in the biosynthesis of 6-thioguanine, which were more intensively expressed in Idared 24 h after inoculation
Summary
Introduction ofE. amylovora cells to apple tree tissue influenced the metabolic pathways of the bacteria. Erwinia amylovora is the causal agent of fire blight, occurring on over 130 plant species belonging to 40 genera, mainly from the family Rosaceae [1] It is a serious bacterial pathogen, causing severe loses in production of apples and pears worldwide. Exopolysaccharides play a role in bypassing the plant defence system, in blocking the vascular system of the plant and in protecting the bacteria against water and nutrient loss during dry conditions and the toxic effect of reactive oxygen species (ROS) [3, 4] They are crucial in the formation of biofilm, which is essential for attachment to several surfaces and for pathogenicity of bacteria [5]
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